1. Field of the Invention
The invention relates generally to a method of and system for defining and photographing computer graphic images, including color images. More particularly, the present invention relates to a method of and an apparatus for defining and photographing computer graphic colored images wherein an exposure mapping table may be used to exchange color resolution for spatial resolution and/or different vertical resolutions may be achieved and/or image storage requirement may be made to depend on exposure time.
2. The Prior Art
In U.S. Pat. No. 4,536,848, entitled "Method and Apparatus for Colored Computer Graphic Photography" of Alice M. d'Entremont et al., granted Aug. 20, 1985, there is disclosed a method of and apparatus for making and photographing computer graphic colored images by defining each computer graphic colored image as a plurality of constant color pixel groups. One of a preselected number of colors is assigned to each pixel group and a photosensitive material is exposed to each constant color pixel group in a predetermined ordered sequence and color. The photosensitive material is constituted by a film, the exposure being effective via respective red, green and blue filters mounted on a rotatable wheel. Images from a black-and-white cathode ray tube are projected onto the film via the filters. The known method involves the steps of first defining and storing within a computer memory the graphic image to be photographed as a plurality of constant color pixel groups. The time during which selected photosensitive material must be exposed to a selected light color intensity for each of the constant color pixel groups is next determined. The determined exposure times are then ordered for each of the constant color pixel groups into either a progressively increasing or decreasing sequence. Each of the constant color pixel groups are thereafter visually displayed at the selected light color and intensity for its determined exposure time to the photosensitive material. The visual display may start with the first of the constant color pixel groups in the ordered sequence and thereafter add each succeeding constant color pixel group in the ordered sequence to the visual display so as to finally display all of the constant color pixel groups simultaneously in the ordered sequence when the ordered sequence involves progressively decreasing exposure times. Alternatively, the visual display may start by simultaneously displaying all the constant color pixel groups in the ordered sequence and thereafter eliminating from the visual display each succeeding constant color pixel group in the ordered sequence so as to finally display only the last of the constant color pixel groups in the ordered sequence when the ordered sequence involves progressively increasing exposure times.
The apparatus and method disclosed in the Letters Patent of d'Entremont, et al. supra, is limited in flexibility. Color and horizontal spatial resolution are fixed. Vertical resolution also is fixed for any given single film recorder. Image storage requirements depend on the number of exposure levels.
Cathode ray tube (CRT) graphics apparatuses have used a technique, well known in the art, for increasing the vertical resolution of a display with an "interlaced" image; an example of this known technique is disclosed in U.S. Pat. No. 4,482,919 entitled "Apparatus for Obscuring Blank Spaces Between Raster Lines and Hard Copies Made from Screen CRT" of Lawrence E. Alston et al. granted Nov. 13, 1984. When an image is interlaced in accordance with this known technique, two fields are required to display the entire image. The first field displays all of the even scan lines and the second field displays all of the odd scan lines. Two vertical scans, then, are required to display the entire image. A disadvantage of this approach is that it may cause an objectionable "flicker" in the display. For this reason, graphics devices have either used a two field interlace as described above or none at all.
The known graphics apparatuses of the type disclosed in the Letters Patent of Alston et al., supra, do not provide the capability to specify more than two fields in an image. The image is either not interlaced or is interlaced as a two-field image, a distinct shortcoming and disadvantage when such devices are used in conjunction with film printers and high resolution is desired.
Of interest as general background information in the field of making and photographing computer graphics colored images is U.S. Pat. No. 4,488,244 of William T. Freeman entitled "Computer Graphic System with Foreground/Background Discrimination" and granted Dec. 11, 1984. The Letters Patent of Freeman, supra disclose a method and system in which each computer graphic image is defined as a plurality of constant color pixel groups. Also of general background interest, is a known method and apparatus in which video signals are converted into a number of two-level brightness distributions on the screen of a CRT. Such a method and apparatus is disclosed in U.S. Pat. No. 4,438,453 of Lawrence E. Alston entitled "Constant Light Greyscale Generator for CRT Color Camera System" granted Mar. 20, 1984.
The architecture of a conventional graphics controller without a color lookup table is illustrated in FIG. 8A. Each display memory location corresponds to one pixel and contains the RGB display values of that pixel. In this type of architecture there is a memory location for each pixel that is displayed, and the contents of this memory location contain the red, green and blue (RGB) color levels which should appear on the display. For example, a display adapter available under the designation Targa.TM. 16 from American Telephone and Telegraph Company, which utilizes this conventional architecture, stores two bytes of information for each pixel. This approach allows five bits for each primary color and hence 32 levels for each primary color. This type of architecture has serious drawbacks for the type of algorithms used in practicing the present invention. The major difficulty is the amount of time required to modify all pixels of a given color since the whole display memory needs to be searched for the required pixels.
A conventional lookup table architecture is illustrated in FIG. 8B. Each display memory location corresponds to one pixel and contains an index into a table. The index selects which of the RGB values in the table will be displayed for a given pixel. Again, there is a memory location for each pixel which is display; however, in this case, the contents of this memory location contain an index into a color lookup table. The contents of this lookup table indicate the color that is to be displayed. A typical example of this type of architecture is the display adaptor available under the designation VDA/D from American Telephone and Telegraph Company. In this known adaptor one byte is allocated to each pixel, giving 256 possible indices into the lookup table. The 256 entries in the lookup table each contain two bytes, allowing five bits for each primary color. If the memory contents for a given pixel contain the number 119, for example, then the color on the display for that pixel is determined by RGB values in entry 119 of the color lookup table. It is easy to modify all pixels of a given color. To change all pixels with index 119 for example, it is only necessary to change the contents of entry 119 in the color lookup table. While more suitable, this approach does not utilize exposure mapping table architecture, as does the present invention.